Have createNewBlock() wait for a tip

- return null on shutdown instead of the last tip
- ignore timeout value node initialization

This allows consumers of BlockTemplate to safely
assume that a tip is connected, instead of having
to account for startup and early shutdown scenarios.

src/node/interfaces.cpp

@@ -1102,6 +1102,9 @@ public:
 
     std::unique_ptr<BlockTemplate> createNewBlock(const BlockCreateOptions& options) override
     {
+        // Ensure m_tip_block is set so consumers of BlockTemplate can rely on that.
+        if (!waitTipChanged(uint256::ZERO, MillisecondsDouble::max())) return {};
+
         BlockAssembler::Options assemble_options{options};
         ApplyArgsManOptions(*Assert(m_node.args), assemble_options);
         return std::make_unique<BlockTemplateImpl>(assemble_options, BlockAssembler{chainman().ActiveChainstate(), context()->mempool.get(), assemble_options}.CreateNewBlock(), m_node);